Recently, as digital communication networks and high quality digital audio systems are developed, there have been proposed increased applications of an analog-to-digital converter (hereafter simply referred to as an A/D converter) which converts an analog signal into a corresponding digital signal. On the other hand, as communication equipments and audio apparatuses have increased and improved functions, a requirement exists for providing A/D converters of small size, low power consumption, wide frequency range and high accuracy. In order to meet the above-mentioned requirement, A/D converters using delta-sigma modulators are being attracted.
The delta-sigma modulator has the basic structure of single integral type, which consists of a noise shaping processing portion and a 1-bit quantization portion. The single-integral type delta-sigma type modulator has the transfer characteristic defined by the following formula (1): EQU Dout=Ain+(1-z.sup.-1)Q (1)
where Dout denotes a digital output signal, Ain denotes an analog input signal, and Q denotes a quantization noise. It is known that the secondary integral type delta-sigma modulator having the secondary transfer characteristic represented by formula (2) is obtainable by cascading two single-integral type delta-sigma modulators. Similarly, the triple integral type delta-sigma modulator having the tertiary transfer characteristic represented by formula (3) can be constructed by cascading three single-integral type delta-sigma modulators. It is also known that as an increased number of cascaded stages, the dynamic range of the modulator is improved, and therefore higher accuracy A/D converters can be realized. EQU Dout=Ain+(1-z.sup.-1).sup.2 Q (2) EQU Dout=Ain+(1-z.sup.-1).sup.3 Q (3)
On the other hand, when the number of single-integral type delta-sigma modulators is simply increased, circuit operation thereof becomes instable, and designed characteristics are not obtained with stability. From this viewpoint, currently, there has been considerable activity in the development of triple integral type A/D converters which have the tertiary transfer characteristic and exhibit stable operation.
However, as is described in detail layer, the conventional triple integral type A/D converters have the disadvantage in that a number of structural elements are necessary to construct the same. This prevents integration of A/D converters. Additionally, the use of a number of structural elements leads to deterioration of the signal-to-noise (S/N) ratio and dynamic range. From the above-mentioned viewpoints, it is desired to provide A/D converters (modulators) having the tertiary transfer characteristic by using a decreased number of structural parts.